It is well known in the art that the brazing of parts of aluminum or aluminum alloys with parts of aluminum, aluminum alloys, copper, steel or titanium can be performed utilizing a lot of different fluxes. A flux very suitable for the brazing of aluminum parts to parts made from aluminum, copper, steel or titanium is based on alkali metal fluoroaluminates. U.S. Pat. No. 6,743,409 describes an alkali metal fluorozincate flux for brazing of aluminum and aluminum alloys. In example 1, a KZnF3 flux is prepared with an X50 value of 3.28 μm. A respective X10 value is not given. US patent application publications 2003/071110 and 2004/164130 disclose a flux suitable for dry fluxing. Table 2 describes a flux having an X(90.93) of 7.5 μm.
Fluxes suitable as brazing flux are commercially available, e.g., from Solvay Fluor GmbH, Hannover/Germany under the tradename Nocolok®.
Several methods are known to apply the flux on the surface of the parts to be brazed.
According to one method, fluxes are applied in dry form by means of electrostatic forces. Such a flux preferably has a certain amount of coarser particles for better pneumatic transport, and a certain amount of fine particles for good adhesion to the metal parts.
According to another method, fluxes are applied in wet form, i.e., in the form of a slurry, to the surface or surfaces. Here, the flux is dispersed in water, organic solvents or mixtures thereof and applied for example by spraying, painting, printing or by immersing the parts into a respective flux preparation. The term “slurry” corresponds to the term “suspension”.
Flux preparations consisting simply of flux and solvent have the disadvantage that a part of the flux preparation often does not adhere strongly to the surface. Either this part is lost as waste, or it must be recycled.
Consequently, in wet applications, binders, for example, polyacrylate or polyurethane, can be applied to improve the adhesion, and thickeners, for example pectines or gelatin, can be applied to reduce the dropping off of the preparation from the parts to be brazed. Both binder and thickener are often organic compounds, and thus, the wet flux preparation contains organic matter which has to be removed prior to the brazing process.
Fluxes for aluminum brazing, including fluxes based on potassium fluoroaluminate, are often manufactured in precipitation reactions. Often, alumina is reacted with HF, provided in the form of hydrofluoric acid, and the resultant fluoroaluminum acid is reacted with a potassium salt, e.g., potassium lye. The precipitated potassium fluoroaluminate is then dried. During drying, potassium fluoroaluminate dust is recovered.